Cavities are frequently formed in semiconductor circuits to reduce coupling of a device such as an inductor, heater, or bolometer to the substrate. Typically a cavity is etched into a substrate material such silicon or SiGe through an opening in a dielectric layer overlying the substrate using a substantially isotropic etch. Typically the substantially isotropic etch, etches the cavity vertically faster than it does laterally. Consequently a very deep cavity may need to be formed to completely remove the substrate laterally from under the device to reduce coupling. The deep cavity may weaken the substrate resulting in breakage and yield loss.
One method to avoid etching a deep cavity is to build an etch stop layer into the substrate under the device with the coupling issue. This method may add significant complexity, cycle time, and cost to the manufacturing flow.